An optical waveguide, such as an optical fiber is formed by a core section transporting the electromagnetic radiation, such as a light beam, and a cladding section that surrounds the core to confine the electromagnetic is radiation to the core. The electromagnetic radiation remains captive in the core by virtue of the difference between the refractive indexes of the core and the cladding sections and their geometries. In an optical fiber, the core section is cylindrical and the cladding surrounding it is tubular and in contact with the cylindrical core.
A Bragg grating is an axial periodical change of the refractive index (n) between the core and the cladding that induces harmonic back reflections of the light beam at a certain wavelength (λ) called the Bragg wavelength. The Bragg wavelength is related to the period length (Λ) of the refractive index change by λ=2nΛ.
Since Bragg gratings have a short period length (Λ) of index change, this periodic index change is usually created by interfering two coherent energy beams to form a stationary energy interference pattern along a section of the core of the waveguide. This stationary energy interference pattern will induce a periodic change in the material structure of the exposed section of the core, leading to the axial periodical change of the effective refractive index (n) between the core and the cladding. A known approach to form a grating in a waveguide, particularly in an optical fiber, is to expose the core of the waveguide to a stationary interference pattern generated by the crossing over of two Ultra Violet (UV) coherent laser beams, where the interference angle dictates the period. The exposure to the, interference pattern initiates semi-permanent material structure changes in the core region. By using proper annealing, one can remove the most unstable part of this semi-permanent material structure changes and obtain, in practice, a permanent grating.
A periodical change of the refractive index in amplitude, as shown in FIG. 1, will create a Bragg grating with a specific reflection spectrum shape, shown in FIG. 2. The percentage of light reflected will follow a Gaussian distribution shape around the Bragg wavelength, with pre-determined side lobe positions and relative levels. As the level of amplitude increases, the Gaussian distribution saturates at the maximal 100% reflection, and the side lobe relative levels increase, as shown in FIG. 3.
A distribution in amplitude of the periodical change of the refractive index, also called apodization, will change the shape, or distribution, of the reflection spectrum of the Bragg grating, as well as the relative levels of the side lobes. FIG. 4 shows a Gaussian type apodization, or axial amplitude profile of the periodical change of the refractive index, and FIG. 5 shows the corresponding reflection spectrum for a saturated Bragg grating. It can be seen that the effect of the Gaussian apodization is to increase the slope of the reflection spectrum and to lower the side lobes level. However, this type of Gaussian apodization produces a non-uniform base value index change, of Gaussian shape, which creates a resonant cavity effect at lower wavelengths and produces one or more undesirable bumps in thee reflection spectrum. In FIG. 4, the base refractive index value is shown as a continous thick line superimposed over the Gaussian type apodization. To compensate for the base level change of the refractive index, one can use a double Gaussian amplitude distribution around the uniform average refractive index, such as shown in FIG. 6. In this instance, the base refractive index value is constant, as shown by the straight continuous thick line in the graph. The associated reflection spectrum, shown in FIG. 7, will then be symmetrical against the Bragg wavelength with sharper slopes and lower side lobe levels.
Currently available methods to create an apodization on a waveguide, such as the one depicted in FIG. 6 are unsatisfactory for a variety of reasons and there is a need in the industry to provide an improved technique and an associated apparatus to perform such operations.